1. Field of the Invention
The present invention relates to an electrically charging method which is capable of mainly charging the surface of a photosensitive material without relying upon the corona discharge, and to an electrically charging device used therefor.
2. Description of the Prior Art
In an electrophotographic method, an image is formed by uniformly charging the surface of a photosensitive material, exposing a document image to light to form an electrostatic latent image that corresponds to the document image on the surface of the photosensitive material, developing the electrostatic latent image and transferring the developed image.
In the above-mentioned method of forming image, in general, the surface of the photosensitive material is electrically charged (mainly charged) by the corona charging accompanied, however, by the generation of ozone which is a cause of environmental pollution. In order to avoid the generation of ozone in recent years, methods have been proposed for mainly charging the surface of the photosensitive material by bringing an electrically conducting rubber roller into frictional contact with the surface of the photosensitive material while applying a bias voltage (Japanese Laid-Open Patent Publications Nos. 149889/1988 and 287887/1989).
According to the above-mentioned charging method based upon the frictional contact, however, the uniformity in the electric charge is lost when foreign matters such as dust, paper powder and the like are trapped between the electrically conducting rubber roller and the photosensitive material and it becomes difficult to stably carry out the electric charging. Moreover, when the surface of the photosensitive material is not perfectly cleaned permitting the toner to stay on the surface, then the remaining toner is adhered to the surface of the photosensitive material during the formation of image causing the photosensitive material to lose its durability. Furthermore, uniform charging of the photosensitive material is not accomplished by the application of a DC bias voltage only unless an AC bias voltage is applied in combination.
In order to solve the above-mentioned problem, the present applicant has previously proposed a method of electrically charging the photosensitive material by bringing a flexible electrically conducting sheet into frictional contact with the surface of the photosensitive material by using an electrically conducting brush roller while applying a DC voltage to the roller (see Japanese Patent Application No. 88148/1992).
This charging method is very meaningful in that since the flexible electrically conducting sheet is depressed by an electrically conducting brush roller to come into intimate contact with the surface of the photosensitive material, the electric charging by friction is uniformly carried out by the application of a low DC bias voltage only without the need of applying an AC bias voltage. According to this charging method in which a charging voltage is applied to the electrically conducting sheet from the ends of the electrically conducting brush, however, the presence of defects such as pinholes in the surface of the photosensitive material permits the electrically conducting sheet to come into contact with the pinholes, so that heavy currents flow into the pinholes from the ends of the electrically conducting brush. Flow of such heavy currents brings about undesirable formation of holes in the electrically conducting sheet to which the ends of the brush are in contact. Accordingly, there develop dot-like high-potential portions causing the photosensitive material to be defectively charged.